Ductwork Sizing Calculator

Background.

This ductwork sizing page is built to derive duct cross-sectional area from airflow and target velocity, then convert that area to round diameter or rectangular width. For width-to-height ratio r, rectangular width is the square root of area times r; the ratio is not multiplied a second time. The implemented convention is “area = airflow ÷ velocity; round diameter = √(4A ÷ π); rectangular width = √(A × width-to-height ratio).”

The editable entries are design airflow, target duct velocity, rectangular duct width-to-height ratio. Use values from the document or measurement that governs this ductwork sizing question; the defaults are only the worked fixture below. Velocity-only sizing does not check friction loss, equivalent length, fittings, static pressure, noise or Manual D balancing. If that ductwork sizing condition is not true, choose a calculation that models the missing convention.

Air Conditioning Contractors of America, Manual D; residential duct-system design documents the convention or governing rule used here. The ductwork sizing output is a transparent scenario under those facts: it does not manufacture an unentered market price, professional determination, carrier quote, legal eligibility finding or locally adopted code value.

What is ductwork sizing calculator?

Ductwork Sizing is the relationship behind this decision: for width-to-height ratio r, rectangular width is the square root of area times r; the ratio is not multiplied a second time. On this page it means area = airflow ÷ velocity; round diameter = √(4A ÷ π); rectangular width = √(A × width-to-height ratio). Velocity-only sizing does not check friction loss, equivalent length, fittings, static pressure, noise or Manual D balancing; that is the line between the reported quantity and a broader hvac analysis.

How to use this calculator.

1. 01Confirm that “area = airflow ÷ velocity; round diameter = √(4A ÷ π); rectangular width = √(A × width-to-height ratio)” matches the ductwork sizing convention you need.
2. 02Replace the fixture values for design airflow, target duct velocity, rectangular duct width-to-height ratio with dated values from the governing record.
3. 03Keep all currencies, measurement units and time periods on the same basis before calculating.
4. 04Read required duct area together with this boundary: Velocity-only sizing does not check friction loss, equivalent length, fittings, static pressure, noise or Manual D balancing.

The formula.

The calculation uses area = airflow ÷ velocity; round diameter = √(4A ÷ π); rectangular width = √(A × width-to-height ratio). In this ductwork sizing model, the entered terms are design airflow, target duct velocity, rectangular duct width-to-height ratio. For width-to-height ratio r, rectangular width is the square root of area times r; the ratio is not multiplied a second time, which is why the relationship is presented under this name rather than as a universal alternative. Velocity-only sizing does not check friction loss, equivalent length, fittings, static pressure, noise or Manual D balancing. Calculations keep full decimal precision through the relationship and round only the returned display values.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Air Conditioning Contractors of America, Manual D; residential duct-system design. Retrieved 2026-08-07. access: open unless marked otherwise.
2. [2]ASHRAE Handbook—Fundamentals, duct design and heat-transfer methods (PAYWALL). Retrieved 2026-08-07. access: open unless marked otherwise.
3. [3]Air Conditioning Contractors of America. Manual J — residential load calculation. Retrieved 2026-08-07. independence: primary; access: open.